CAES, thermodynamics, efficiency and exergy (part 1)

I thought that I would write a post about CAES and a couple of issues that I feel are commonly misunderstood. This post has been inspired by things that I have heard at academic conferences and things that I have read in both academic and non-academic literature. I also thought that I would share a couple of insights about conventional CAES which have been passed down to me.

A couple of notes about conventional CAES

Conventional CAES is an energy storage technology that has been around for several decades. It is interesting because although there are two plants currently functional and in existence, no new plant has been built in the last 20 years, despite the fact that both of the existing plants remain open and continue to function economically. This can probably be attributed to high CAPEX costs for CAES and other cheaper generation technologies which represent similar or better investments, added with an uncertainty of how to class CAES and view its efficiency.

Diabatic CAES Figure 1: The convential diabatic CAES system with a  recuperator. Natural gas is mixed with the compressed air in the generation unit.

Calculating the efficiency of CAES facilities is perhaps not as straightforward as it first seems. The McIntosh CAES plant uses 1 kWh of natural gas and 0.69 kWh of electricity to produce 1 kWh of peak electricity. The energy efficiency in terms of energy-output/energy-input is then around 59%, i.e. quite low for an energy storage technology. However, if instead you consider that the efficiency of a conventional thermal gas generator is around 40%, you would only ever get 0.4 kWh of electricity out of the 1 kWh of gas used in the CAES plant. This makes the efficiency look much better, as now it effectively appears as though you put 0.69 kWh + 0.4 kWh = 1.09 kWh of electricity in and you get 1 kWh of electricity out, giving an efficiency of 92%. Conversely, another argument would be that the 1 kWh of electricity required 2.5 kWh of gas to generate, and hence the energy input is 3.5 kWh of gas to produce 1 kWh of electricity, giving a much poorer efficiency of 29%.

The point of all this is that the “efficiency” values often quoted for CAES must be treated with caution and are generally not comparable with other storage technologies which input and output electricity only, as CAES plants are NEITHER purely energy storage NOR thermal generation, but in reality they represent a mix of both. I haven’t quite decided how to interpret this myself except that when considering CAES as an energy storage option, it is more important to consider from what source the electricity used in charging comes from than other energy storage technologies. For example, using CAES in the context where it would mainly have an electricity-from-renewable input could be regarded as boosting the efficiency of gas generation and hence a good thing under these circumstances, whereas using CAES as a way to store fossil fuel generated electricity would seem like a bad idea. I don’t fully endorse this last statement, rather I’m just using it as an illustration…

Keep an eye on the blog for part 2.

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